Why my sshd stopped listening after a port change
Recent Ubuntu releases start OpenSSH through ssh.socket. The socket unit, not sshd_config, owns the listening address, so changing Port alone can leave you with a daemon that never sees a connection.
Two ways out: keep socket activation and override ListenStream in a drop-in (mind BindIPv6Only), or disable the socket and run the plain service:
systemctl disable --now ssh.socket systemctl enable --now ssh.service
Whatever you choose, test from a second session before closing the first one.